1. Field of the Invention
The present invention relates to an image recording material having a photo/thermosensitive layer on a support, which photo/thermosensitive layer increases solubility thereof to an alkaline developer as a result of exposure to light. More specifically, the present invention relates to a so-called “direct plate-making” positive-type image recording material, capable of direct plate-making by scanning a high-energy density laser beam, such as an infrared laser, on the basis of digital signals from computers or the like.
2. Description of the Related Art
In recent years, progress in lasers has been rapid, and, in particular, solid state and semiconductor lasers with higher outputs and smaller in size; with emission wavelengths in a range between the near-infrared region and the infrared region, have become more readily available. A method whereby the solubility of the photosensitive resin is changed by exposure to light, to record images thereon, by using one of these high-output lasers (usually, having an exposure energy density of more than 5 to 10 kW/cm2) is called a heat-mode or a thermal recording method. In the planographic printing field, these methods have attracted attention recently as recording methods for direct plate-making based on digital data from computers and the like.
More specifically, products produced by the aforementioned method and sold in the market include thermal positive-type planographic printing plate precursors having on a support a photosensitive layer that becomes more soluble as a result of exposure to high-output lasers (hereinafter, referred to as positive-type photosensitive layers) and thermal negative-type planographic printing plates having on a support a photosensitive layer that becomes less soluble as a result of exposure to high-output lasers (hereinafter, referred to as negative-type photosensitive layers).
Positive image recording materials for direct plate making using infrared lasers are image recording materials including both a substance that generates heat by absorbing light and a positive-type photosensitive compound, such as a quinone diazide compound or the like, dispersed together in an alkali-soluble resin. In such image recording materials, images are formed by a mechanism in which the positive-type photosensitive compound functions substantially as a solubilization-suppressing agent which reduces the solubility of the aqueous alkaline solution-soluble resin (which resin will hereinafter be referred to as “an alkali-soluble resin”) in the image portions, while, in the non-image portions, as a result of decomposition by heat, the compound becomes unable to exert the same solubilization-suppressing effect, whereby resin in the non-image portions is removed in the process of developing and images are formed. Hitherto, various alkali-soluble resins and solubilization-suppressing agents have been proposed (e.g., see Japanese Patent Applications Laid-Open (JP-A) Nos. 7-285275 and 10-268512, and Japanese Patent Application National Publication (Laid-Open) No. 11-506550).
However, photosensitive compositions, which are known in the art, and which have been used as recording layers of known thermal positive-type planographic printing plate precursors, have a problem, insofar that difference in the solubility thereof to a developer between the exposed and nonexposed portions (which difference will hereinafter be referred to, for the sake of convenience, as “discrimination in solubility”) is insufficient, whereby excess development or insufficient development is caused depending on variations in the application conditions. This results in insufficient contrast of images after exposure and development.
Specifically with regard to conventional positive-type image forming materials, images have been formed by modifying the alkali solubility of alkali-soluble resins, mainly by means of thermal breakage of electrostatic interactions between resins and solubilization-suppressing agents, and by removing, in the course of development exposed portions in which resins have become more soluble. Because changes in alkali solubility are produced merely by modest changes in structure caused by heat, such image forming materials have inevitably entailed problems of poor discriminations in solubility, and inferior image contrast.
In order to improve discriminations of images formed by the positive-type planographic printing plate materials, for example, a method of adding a phenolic hydroxyl group-containing compound has been proposed (e.g., see JP-A No. 2000-241966). However, although the phenolic hydroxyl group-containing compound makes removal of the non-image portion by the alkali developer easy (i.e., achieves enhanced solubility of the non-image portion to the alkali developer), the compound also enhances solubility of the image portion, which results in deterioration in sharpness of images. This tendency is conspicuous, in particular, in regions of images like fine lines and dots which take up a relatively small percentage of the image portion. In view of this, the conventional phenolic hydroxyl group-containing compound needs to be improved especially in contrast.